An optical fiber is manufactured by drawing an optical fiber preform made of a glass. Before the drawing, in general, a glass body called “dummy glass body” is melt-spliced to the optical fiber preform. Next, a predetermined jig is attached to the dummy glass body, and a coupled body of the optical fiber preforms and the dummy glass body is suspended within a spinning furnace. In this manner, in the state where the coupled body of the optical fiber preform and the dummy glass body is suspended, at least a portion of the optical fiber preform is heated to be drawn, so that the optical fiber is manufactured. In addition, even in the process of manufacturing the optical fiber preform, there is a case where a method of splicing a glass porous body to the dummy glass body made of a glass is performed. As an example, there is a VAD (Vapor-phase Axial Deposition) method.
When the optical fiber is manufactured in this manner, the optical fiber preform and the glass body called “dummy glass body” are processing objects, which are sometimes referred to optical fiber works. For example, in the case where the above-described optical fiber preform and dummy glass body are melt-spliced to each other, each of the optical fiber preform and the dummy glass body is referred to as an optical fiber work; and in the case where the optical fiber preform is drawn, the coupled body of the optical fiber preform and the dummy glass body is referred to as an optical fiber work. In addition, in a process of manufacturing an optical fiber preform, a coupled body of a dummy glass body and a glass porous body, or the dummy glass body is referred to as an optical fiber work.
As described above, when the optical fiber preform is manufactured, when the optical fiber preform and the dummy glass body are melt-spliced to each other, when the optical fiber preform is drawn, or the like, since at least a portion of the optical fiber work is heated, the optical fiber work is subjected to thermal shock. In this manner, when the optical fiber work is subjected to the thermal shock, in some cases, crack occurs in the optical fiber work. If the crack occurs, in some cases, the manufactured optical fiber may be disconnected. Therefore, when abnormality such as crack occurs in the optical fiber work, rapid treatment is needed.
The following patent document 1 described below discloses a method of inspecting abnormality such as crack by using a laser without destruction of an inspection object. In the inspection method, occurrence of crack or a position of crack is identified by a Raman scattered light intensity distribution of the inspection object at a focus position of a laser beam.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-247943